Seasonal dynamics of Sea Surface Salinity off Panama : the Far Eastern Pacific Fresh Pool

G. Alory1, C. Maes2, T. Delcroix2, N. Reul3, S. Illig2,4

1LEGOS, OMP, CNAP, Université de Toulouse, Toulouse, France2LEGOS, IRD, Université de Toulouse, Toulouse, France3LOS, IFREMER, Plouzané, France4IMARPE, Callao, Peru

Vienna, Austria, 22-27 April 2012

Why focus on SSS in this region?

mean SST

mean SSS

SSS seasonal EOF

• Between 2 climate relevant features: Eastern Pacific warm pool and equatorial cold tongue

• Minimum in SSS (<33: Far Eastern Pacific Fresh Pool) and maximum seasonal variability

• Strong air-sea-land interactions in this region: monsoon, gap winds... (e.g. Xie et al. 2005, Fiedler and

Talley 2006, Kessler 2006)

• Potentially active role of salinity stratification on regional climate (de Boyer Montegut et al. 2007)

• Good test ground for new SSS satellite products (SMOS, Aquarius)

Delcroix et al., 2011

°C

Main SSS data source: Voluntary Observing Ships

Transect snapshots

• Steep SSS fronts (up to 4 pss/1°) at Fresh Pool west/east boundaries with seasonal displacement >1000 km

• Not always related to SST fronts

Well-sampled TSG line from Panama canal to Tahiti

1950-2009 obs. density

per ¼

°b

in

April

Dec.

SSS SST

Along-track SSS/SST climatology

• Life cycle of the Fresh Pool : -June-Dec: building

- Jan-Mar: westward stretching

- Apr-May: destruction

• SSS<33 generally associated with SST>28°C

• Mar-Apr coastal SSS increase and cooling in phase

°C

Central American monsoon

Seasonal north-south migration of ITCZ + high coastal topography:

• in summer, south-westerly winds and strong rainfall in Panama Bight

• in winter, strong north-easterly winds through mountain gaps (Papagayo, Panama) and low rainfall

QScat Wind+

TRMM Rainclimatology

20

15

10

5

0m/s

mm/d

ITCZ

Wind-driven Ekman pumping

D20

m/d

2.5

0

-2.5

• Upward pumping under the ITCZ

• Winter Ekman pumping dipoles driven by gap winds

• Winter upwelling of salty waters

• Barrier layer under Fresh PoolD20

T-mxl

ρ-mxl

J F M A M J J A S O N D

0

40m

80m

35

33

30

S(z)Argo

Sea Level Anomaliescm

• Winter eddy dipoles

• Slow south-westward propagation

• SLA trough at 10°N = Costa Rica dome

• Associated with geostrophic currents

15

0

-15

Surface currents

NECC

SEC

0.8

0.6

0.4

0.2

0

m/s

• Eastern end of the equatorial current system: South Equatorial Current (SEC), North Equatorial Counter-Current (NECC)

• NECC closely associated with ITCZ

• Winter gap winds create coastal cyclonic/anticyclonic circulation

Along track advection

• JJAS: eastward NECC keeps Fresh Pool trapped to the coast and maintains strong SSS front• JFM: westward stretching of Fresh Pool by SEC• FMA: upwelling in Panama Bight propagating S-E

SEC

NECC

Main Mechanisms

Surface Currents JFM

• Summer rain � building of Fresh Pool, SSS minimum • Winter gap wind � Westward advection of Fresh Pool by SEC

Upwelling � destruction of Fresh Pool

Wind/Rain JAS

Panama gap wind

Upwelling

NECC

SEC

Alongtrack regressionbetween TSG/SMOS

colocated dataSignal=10*Error

SMOS: seasonal+ intraseasonal+ interannualvariability

- Main seasonal features appear

- Land contamination near coastline

In situ Climatology vs SMOS 2010 (weekly, ¼°)

SMOS validation

SMOS 2010: monthly maps of Fresh Pool

SMOS captures main seasonal features of Fresh Pool, can monitor itsspatial extension and associated SSS fronts

Jan Feb Mar

Apr May Jun

Jul Aug Sep

NovOct Dec

Conclusion and Perspectives

• The Far Eastern Pacific Fresh Pool (SSS<33) is a quasi-permanent feature with the lowest mean SSS and maximum seasonal SSS variations in the tropical Pacific

• Its seasonal cycle results from strong regional ocean-atmosphere-land interactions (monsoon)

• New SMOS satellite is able to monitor the Fresh Pool and complement in situdata

JGR paper out now!

• To quantify the relative contribution of different processes onthe SSS seasonal cycle, regional modelling would be useful

• Potential role of vertical salinity stratification on the regional climate has to be explored